TANKERS used for liquid fuels are classified according to their capacity.
In 1954 Shell Oil developed the average freight rate assessment (AFRA) system which classifies tankers of different sizes. To make it an independent instrument, Shell consulted the London Tanker Brokers’ Panel (LTBP). At first, they divided the groups as General Purpose for tankers under 25,000  tons deadweight (DWT); Medium Range for ships between 25,000 and 45,000 DWT and Large Range for the then-enormous ships that were larger than 45,000 DWT. The ships became larger during the 1970s, and the list was extended, where the tons are long tons:
10,000–24,999 DWT: General Purpose tanker
25,000–44,999 DWT: Medium Range tanker
45,000–79,999 DWT: Large Range 1 (LR1)
80,000–159,999 DWT: Large Range 2 (LR2)
160,000–319,999 DWT: Very Large Crude Carrier (VLCC)
320,000–549,999 DWT: Ultra Large Crude Carrier (ULCC)